Method of producing bright electroplate on electropolished surfaces



Patented Aug. 7, 1951 METHOD OF PRODUCING BRIGHT ELEC- TROPLATE N ELECTROPOLISHED SURFACES Charles L. Faust and John G. Beach, Columbus,

Ohio, assignors, by mesne assignments, to Battelle Development Corporation,

Columbus,

Ohio, a corporation of Delaware No Drawing. Application June 25, 1948, Serial No. 35,303

1 Claim. 1

This invention relates to a method for treating a metal surface in preparation for subsequent plating. In particular, it relates to treating passive metallic surfaces, 1. e., surfaces which are difficult to plate, with an adherent coating of metal due to a film or scale thereon making the surface electrochemically inactive.

A metallic surface may be passivated due to electropolishing, from oxidation in air, and from oxide scale formation, resulting during heat treatment or annealing as, for example, of beryllium-copper alloys.

For example, it is well known that, after electropolishing, certain metals, such as brass, nickel-silver, aluminum, copper, and steel, have a surface condition which retards tarnishin and discoloration. This passive metallic surface, or protective effect, apparently is associated with a salt or oxide coating on the surface of these metals. While the coating is sufliciently thin to be transparent, it is thick enough to offer substantial protection, to provide good insulation against electrochemical plating, and to prevent good adherence by metals subsequently plated on such passive or protected metal surfaces.

Electropolished brass, for example, can be bright plated in commercial bright nickel plating solutions, and the resulting plate has a very attractive mirror-like appearance however, in many cases the bright nickel plate does not adhere satisfactorily to the electropolished brass surface. It is well known in the art that brass, which has been treated in solutions containing chromic acid, must alsobe activated before it can be acceptably bright plated with other metals. dipped in many types of solutions to effect good adherence of subsequently eleetrodeposited metal coatings, but the known dips destroy the color and luster of an electropolished surface by their It is also known that brass may be etching eifect, producing streaks, discoloration,

splotches, etc., as a result of non-uniform dissolution of the surface layer of brass. Thus, it is apparent that with these prior art methods it is impossible to apply a plate having both acceptable luster and adherence.

The oxide scale formedon the surface of beryllium-copper alloy during heat treatment has required expensive steps in the past for its re moval and has thus increased the manufacturing cost of components made from this alloy.

This scale has been removed in industry by using various concentrations of sulfuric acid. One shop practice used in cleaning'work from precipitation-hardening heattreatments of Be Cu alloys is to immerse the piece in 10% sulfuric acid nickel-silver, aluminum, copper,

plate on beryllium-copper alloys.

solution at room temperature for a period of 20 to minutes. The piece is then rinsed in water and given a quick immersion in dilute potassium dichromate solution at room temperature fora period from a few seconds to one minute. The dichromate is the active agent, but it is also the expensive ingredient, and it is quite susceptible to reduction by dirt and dust, as well as by the metal. It is, therefore, necessary in order to obtain best results to make up the solution fresh just before it is needed.

The straight sulfuric acid solutions only remove the oxide coating and do not adversely aifect the dimensions of the alloy. Immersion time, therefore, is of no importance with these solutions; however, the final finlshesare not bright, but are dull and patchy. 0n the other hand, the expensive diohromate solution dissolves the metal and can upset close dimensional tolerances which are requisite in the small parts for which beryllium-copper alloys are generally used.

Electroplate finishes are applied to berylliumcopper for a number of purposes. Nickel plating over beryllium-copper alloy providesnormal protection from oxidation and the retention of a neat light-colored appearance. A nickel-chromium plate is preferred for assuring complete resistance from corrosive action over long periods. Gold or silver may be employed or a combination of silver and rhodium, in the fabrication of electrical parts, particularly, if resistance to wear is necessary, as it must be on moving contacts. These and other electroplate finishes necessitate particular attention to the cleaning of the surfaces of Be-Cu components, especially after heat treating or annealin -which forms oxide scale on metal surfaces.

It is an object of this invention to provide a process for the activation of electropolished metallic surfaces, and for the removal of scale which has been formed in heat treating metals. g

It is another object of this invention to pm, vide a method for producing an adherent plate on bright dipped and/or .electropolished brass,

and steel surfaces.

It is a further object of the invention to provide a method whereby the removal of scale formed during annealing or heat treating beryllium-copper alloys may be effectively accomplished.

It is a still further object of this invention to provide a method for producing an adherent It has now been discovered that simple chemi cal dips may be used for activating metal surfaces to receive an adherent bright metal plate without, in any way, damaging the appearance of the plated product. Moreover, electropolished metals may be dipped without loss of relative smoothness and mirror-like luster, so that subsequent bright metal plates can attain full color and luster, as would be expected from deposition on a well polished metallic surface. It has been found that these chemical dips are effective, too, in the removal of oxide scale on heat-treated beryllium-copper alloys.

It has been found that dilute oxalic acid solutions, when properly inhibited by a less reactive acid, which will be more fully described herein below, can satisfactorily activate bright dipped and electropolished brass, nickel-silver, aluminum, copper and steel surfaces without destroying the brilliance of the polish, so that a'subsequent metal can be deposited thereon with full color and brilliance. Dilute oxalic acid solutions, when inhibited by a less reactive acid, effectively remove the oxide scale formed when heat treating beryllium-copper alloys.

Sulfuric acid is a suitable inhibiting or retarding acid. Also, it has been found that organosulfonic acids are suitable retarding acids. For example, mixed alkane sulfonic acids, having the general formula designated as RSOsH, are very satisfactory; in this formula, R is an alkyl radical of the homologous series CnH21L+1 such as methyl, ethyl, propyl, butyl, etc. In many metal applications, these ,are strong acids and are liquids miscible Iwithwater. It has also been discovered that aromatic sulfonic acids are suitable retarding acids for use with oxalic acid. For example, metabenzene disulfonic acid, monobenzene sulfonic acid, and others, such as the naphthalene sulfonic acids may be used. Also, in the category of aromatic sulfom'c acids, substituted sulfonic acids may be used, such as toluene sulfom'c acid, xylene sulfonic acid, etc. It is possible to use mixtures of sulfuric acid and organo-sulfonic acids. Their principal effect in the bath is to condition the action of the oxalic acid to provide for uniform removal of the superficial coatings on the electropolished metals or scale on heat treated metals, without destruction of the bright polished surface appearance or causing other surface damage. It is believed that these retarding acids prevent a selective etch by the oxalic acid after it has successfully removed the superficial salt or oxide film, so that each crystal face is made equally active toward receiving the bright metal plate.

To illustrate the invention in greater detail, as it applies to the activation of electropolished surfaces prior to electroplating, the following examples are cited. After any of the following surface activating sequences, a bright metal may be plated on the basis metal by any of the commercially known processes to obtain excellent color and mirror brilliance. r I

Example 1 Brass, nickel-silver, aluminum, copper, beryllium-copper, and steel stripswere electropolished and rinsed to remove the electropolishing bath solution. The strips were then immersed for minutes at 100 to 170 F. in a sulfuric acid solution (by volume of 1.84 specific gravity acid) containing 0.5 to 10 oz./gal. of oxalic acid. The strips were finally rinsed and bright nicke plated.

4 Example 2 A brass strip was electropolished and rinsed. It was then immersed for 5 minutes at 160 to 170 F. in a solution containing 38 oz./ga1lon of metabenzene disulfonic acid, and 1 oz;/ gallon of oxalic acid (specific gravity of the resulting sulfonic-oxalic acid solution being 1.13). After immersion, the strip was rinsed, and bright nickel plated.

Example 3 A brass strip was electropolished and rinsed. It was then immersed for 5 minutes at 160 to 170 F. in a solution containing 55 oz./gallon of metabenzene disulfonic acid and 2 oz./gallon of oxalic acid (the specific gravity of the resulting solution being 1.22). The strip was given a final rinse and bright nickel plated.

Example 4 A brass strip was electropolished and rinsed to remove any of the electropolishing bath solution remaining on the surface of the brass. It was then immersed for 1 minute at 160 F. in a solution containing 50 to per cent by volume of mixed alkane-sulfonic acids (methyl, ethyl, propyl, etc. sulfonic acids having the formula RSOaH where R is an alkane of the series CnH2n+l). Oxalic acid was added to this solution in the amount of 1.5 to 30 oz./gallon of the 90 per cent (by volume) solution. The strip was finally rinsed and bright nickel plated.

Example 5 A brass strip was electropolished and rinsed. It was then immersed in a solution containing g./1. of oxalic acid. (This particular dip can be used in the case of a bright nickel plating solution which deposits a small amount of a second metal, i. e., deposits alloy bright plates.) The strip was finally rinsed and bright alloy nickel plated.

Example 6 A brass strip was electropolished and then rinsed. It was immersed for 1 to 5 minutes at to F. in 10% sulfuric acid solution (by volume of 1.84 specific gravity acid) containing 0.5 to 10 oz./gal. of oxalic acid. The strip was finally rinsed and bright silver plated.

Example 7 Two strips of nickel-silver alloy were electropolished and rinsed. They were then immersed for 1 to 5 minutes at 160 to 170 F. in 10% sulfuric acid solution (by volume of 1.84 specific gravity acid) containing 0.5 to 10 oz./gal. of oxalic acid. The strips were finally rinsed and one was silver plated and the other was gold plated.

Example 8 Brass, nickel-silver and steel strips were electropolished and rinsed. The strips were then immersed for up to 5 minutes at 120 to 200 F. in 10% sulfuric acid (by volume of 1.84 specific gravity acid) containing 0.5 to 10 oz./gal. of oxalic acid. The strips were finally rinsed and bright copper plated.

The operating ranges in the several examples given above may be quite broad. The time of dippingcan be varied from A to 10 minutes, and the temperature can be varied from room temperature to 220 F. At the higher temperatures shorter dipping time is generally required.

To illustrate the effectiveness of this invention, bright nickel deposited with good appearance on electropolished brass can be loosened easily and stripped off in one continuous piece, without detriment to the electropolished brass surface. When the electropolished brass is treated according to Example 1, above, using a 5 minute dip at 160 F. in the sulfuric acid solution, containing 8 oz./gal. of oxalic acid and then bright nickel plated as shown, the plate is sufiiciently well adherent to withstand bending, twisting, filing, etc., without separation from the electropolished brass.

It is seen that quite a wide range of chemicals and operating conditions can be used in solutions containing oxalic acid as the principal activating chemical. In the case of Example 5, where certain special bright nickel plating baths are to be used, the retarding acid is not needed in the activating dip along with the oxalic acid. Although it is believed that the oxalic acid does not clean the entire surface when used without an inhibiting acid, in this particular case, the subsequent fine grain alloy plate deposited after the dip has satisfactory luster and adherence.

In all examples, where the step refers to rinse and bright nickel plate, any commercial plating process may be used to apply other bright metals such as copper, silver, gold, tin, zinc, cadmium, etc. Other bright metals can be electrodeposited with good adherence on electropolished metals that have been treated according to the method of this invention.

These examples should in no way be construed as to limit the invention to a method for activating an electropolished surface, for the method and solutions cited are also applicable to the removal of oxide scale from beryllium-copper al- 10y. A preferred solution composition for a dip to efifectively remove the oxide scale from berylhum-copper alloy is as follows: 10% sulfuric acid '6 solution containing 1.5 oz./gallon of oxalic acid. The metal is dipped in this solution, whose temperature is about F. for from 3 to 5 minutes. The metal surface may then be rinsed and bright metal plated.

In summary this invention resides in the discovery that a solution of oxalic acid and a retarding acid, as described herein, are particularly advantageous, in the removal of the passivity of metallic surfaces, i. e., in removing a superficial coating of a salt, oxide film, or scale on surface of the basis metal in order to prepare it to receive a subsequent plating which is bright, not streaked nor discolored, and which is adherent to the basis metal.

What is claimed is:

The method of producing an adherent bright nickel plate on an electropolished brass surface which comprises rinsing the aforesaid brass surface, immersin the rinsed brass surface in a 10% sulfuric acid solution containing 0.5 to 10.0 ounces per gallon of oxalic acid, rinsing the brass surface again, and finally plating said brass surface with a bright nickel plate.

CHARLES L. FAUST. JOHN G. BEACH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,334,092 Harmeling Mar. 16, 1920 1,954,745 Peterson .4... Apr. 10, 1934 2,176,389 Brandt Oct. 1'7, 1939 2,383,434 Hovis Aug. 21, 1945 2,401,738 Diffenderfer June 11, 1946 2,430,435 Sperry Nov. 4, 194;? 

